Medical treatment of disorders caused by abnormal organ function typically employ pharmaceutical agents designed for either compensating for such abnormal organ function or treating the dysfunctional organ tissue. However, in some cases, pharmaceutical therapy cannot be instated since organ function is oftentimes complex and/or not completely understood.
In such cases, the only viable alternative is surgical replacement of the non-functional organ, which is now widely used for treatment of liver and kidney failure, both acute and chronic, as well as for cancer and certain inborn abnormalities. However, the need for donor organs far exceeds the supply. Organ shortage has resulted in new surgical techniques, such as splitting adult organs for transplant. Despite fairly good results, such techniques still suffer from a lack of donor tissue.
The lack of viable donor tissue has led to the emergence of stem cell replacement therapy, which relies on stem cell plasticity i.e., the ability to give rise to cell types in a new location that are not normally present in the organ in which the stem cells are located.
Stem cells are generally classified according to their origin, essentially adult, embryonic or neonatal origin. Embryonic stem cells deriving from the inner cell mass of the blastocyst are pluripotential, bring capable of giving rise to cells found in all three germ layers. Despite long held belief adult stem cells are not as lineage restricted as previously thought. In particular, haematopoietic and neural stem cells appear to be the most versatile at cutting across lineage boundaries. For example, recent reports suggest that hematopoietic stem cells (HSCs) of human origin have a hepatic potential. Studies of liver or bone marrow transplantation from sex mismatched donors, identified bone marrow-derived hepatocytes in recipients [Alison (2000) Nature 406:257; Theise (2000) Hepatology 32:11-16; Korbling (2002) N Engl J Med 346:738-746]. Murine and rat HSCs were also found to migrate to irradiated or injured adult livers, and to differentiate into hepatic cells [Petersen (1999) Science 284:1168-1170; Theise (2000) Hepatology 31:235-240; Lagasse (2000) Nat Med 6:1229-1234]. Furthermore, single murine hematopoietic stem cell transplantation has resulted in detection of HSC-derived cells in the liver of irradiated recipients with a low percentage of transplanted cells exhibiting immunohistochemical and morphologic properties of hepatic epithelial cells [Krause (2001) Cell 105:369-377].
The mechanisms that guide circulating hematopoietic stem cells are clinically significant because the success of stem cell transplantation depends on efficient targeting (also referred to as homing) of grafted cells to the recipient target tissue [Mazo and von Adrian (1999) Journal of leukocyte Biology 66,25-32]. It is due to this homing of transplanted cells that bone marrow transplantations do not require invasive surgery, as in the case with the transplantation of any other organ, but rather can be effected by simple intravenous infusion.
Homing of HSCs can be defined as the set of molecular interactions that allows circulating HSCs to recognize, adhere to, and migrate across bone marrow endothelial cells resulting in the accumulation of HSCs in the unique hematopoiesis-promoting microenvironment of the bone marrow. Homing of progenitor cells can be conceived as a multi-step phenomenon [Voermans (2001) J. Hematother. Stem Cell Res. 10:725-738, Lapidot (2002) Leukemia 16:1992-2003]. HSCs arriving to the bone marrow must first interact with the luminal surface of the bone marrow endothelium. This interaction must occur within seconds after the HSCs have entered the bone marrow microvasculature and provide sufficient mechanical strength to permit the adherent cell to withstand the shear force exerted by the flowing blood. Adherent HSCs must then pass through the endothelial layer to enter the hematopoietic compartment. After extravasation, HSCs encounter specialized stromal cells whose juxtaposition supports maintenance of the immature pool by self-renewal process in addition to lineage-specific HSCs differentiation, proliferation and maturation, a process that involves stroma-derived cytokines and other growth signals.
Only a limited number of factors involved in stem cells homing are known to date; these include, the ligand for c-kit, stem cell factor, which has been shown to play a central role in adherence of HSCs to the stroma [Peled (1999) Science 283:845-848]; and integrin interactions (e.g., β1-Intergrins), which were shown to be crucial to the migration of HSCs to the foetal liver [Zanjani (1999) Blood 94:2515-2522].
One important molecular interaction which is considered central to HSC homing is that of chemokine stromal derived factor (SDF-1) and its cognate receptor, CXCR4.
SDF-1 is the only known powerful chemoattractant of hematopoietic stem cells of both human [Aiuti (1997) J. Exp. Med. 185:111-120] and murine origin [Wright (2002) J. Exp. Med. 195:1145-1154] known to date. SDF-1 is widely expressed in many tissues during development [McGrath (1999) Dev. Biol. 213:442-456] and adulthood [Nagasawa (1994) Proc Natl Acad Sci USA 91:2305-2309; Imai (1999) Br J Haematol 106:905-911; Pablos (1999) Am J Pathol 155:1577-1586], such as for example the liver [Shirozu (1995) Genomics 28:495-500; Nagasawa (1996) Nature 382:635-638; Goddard (2001) Transplantation 72:1957-1967]. Previously, the present inventors were able to show that bone marrow homing and repopulation by sorted human CD34+/CD38−/low stem cells transplanted into the tail vein of irradiated immune deficient NOD/SCID and NOD/SCID/B2m null mice, are dependent on SDF-1/CXCR4 interactions [Peled (1999) Science 283:845-848; Kollet (2001) Blood 97:3283-3291]. More recently, the present inventors also established a role for these interactions in G-CSF-induced mobilization of murine and human stem cells [Petit (2002) Nat Immunol 3:687-694].
In view of the ever-expanding use of stem cell therapy, it is highly desirable to further elucidate the mechanism behind stem cell homing and target repopulation in order to improve the efficiency and success rate of cell replacement therapy.
Hepatocyte growth factor (HGF), initially identified as a potent mitogen for mature hepatocytes, is a kringle-containing polypeptide growth factor sharing structural homology with plasminogen [Nakamure (1984) Biohcem. Biophys. Res. Commun. 122:1450; Nakamura (1987) FEBS Lett. 224:311; Gohda (1988) J. Clin. Invest. 81:414; Zanegar Cancer Res. 49:3314; Nakamura (1989) Nature 342:440].
HGF is a mesenchyme-derived pleiotropic factor which regulates cell growth, cell motility and morphogenesis in various types of cells [Matsumoto (1993) Goldberg I D, Rosen E M (eds): Hepatocyte Growth Factor-Scatter Factor (HG-SF) and C-met Receptor, Basel, Switzerland, Birkhauser Verlag, 1993, 225; Gherardi (1990) Nature 346:228; Weidner (1990) J. Cell Biol. 111:2097; Higasho (1990) Biochem. Biophys. Res. Commun. 170:397; Rubin (1991) Proc. Natl. Acad. Sci. USA 88:415]. C-met proto-oncogene is the natural and only receptor for HGF known to date. HGF is considered a humoral mediator of epithelial-mesenchymal interactions responsible for organogenesis of various tissues and organs, regeneration of organs and growth, invasion and metastasis of tumor cells [Matsumoto (1996) J. Biochem. 119:591]. In the hematopoietic system, HGF augments the growth of hematopoietic progenitor cells [Kmiecik (1992) Blood 80:2454; Nishino (1995) Blood 85:3093; Mizuno (1993) Biochem Biophys Res. Commun. 194:178 Galimi (1994) J. Cell Biol. 127:1743). Interestingly, an acute liver injury has been reported to trigger expression of HGF as determined by in-situ hybridization of HGF after stimulation of rat liver with carbon tetrachloride [CCl4, Armbrust (2002) Liver 22:486-494].
Information pertaining to the interaction of HGF with hematopoietic cells is incomplete, although a possible role in hematopoiesis has been suggested. HGF was found to be constitutively produced by human bone marrow stromal cells [Takai (1997) Blood 89:1560-1565]. Recently, HGF was found to be synergistic with GM-CSF and IL-3 in proliferation of murine myeloid progenitor cell line and murine hemopoietic progenitor cells (HPCs) enriched from bone marrow (BM) or fetal liver [Kmiecik (1992) Blood 80:2454-2457; Mizuno (1993) Supra; Nishino (1995) Blood 85:3093-3100]. In addition, a synergistic proliferative effect of HGF with other growth factors on human HPCs has been observed and expression of C-met on CD34+ HPC was detected as well [Galami (1994) J. Cell Biol. 127:1743-1754; Goff (1996) Stem Cells 14:592-602; Weimer (1998) Exp. Hematol. 26:885-894].
While reducing the present invention to practice the present inventors have uncovered that HGF can upregulate CXCR4 expression and promote SDF-1/CXCR4 dependent stem cell motility and migration to the target tissue. These findings provide a novel approach for sensitizing stem cell recruitment to a target tissue and as such can be used in various cell and tissue replacement protocols.